1. Technical Field
The present invention relates generally to satellite communication systems and more particularly to a satellite system and method of operation for delivering multicast messages to a plurality of subscriber units.
2. Related Art
Satellite communication systems are generally known to facilitate wireless communication with mobile terminals across most areas of the Earth. The satellite communication systems typically provide wireless coverage in areas served neither by the public switched telephone network (PSTN) nor by cellular communication systems and thus provide extremely valuable functions. Satellite communication systems are also valuable for international travelers because location updates and call routing are simplified. For example, a local communication network does not have to be compatible with the network of the international traveler's location to route a call.
In a satellite communication system, at least one satellite operates from an orbit above the earth. International telecommunication satellites typically operate from a geostationary orbit (GEO) that is approximately 36,000 kilometers above earth. Alternatively, telecommunication satellites have also been deployed in orbits closer to Earth such as low Earth orbits (LEOs) at an altitude from 780 kilometers to 1,390 kilometers or medium Earth orbits (MEOs) at approximately 10,000 kilometers. A network control center serves as a terrestrial connection point for communications in the satellite communication system. The network control center transmits communications to and from the subscriber units via one or more satellites. Typically, a single network control center services a large geographic area. In some cases, this geographic area extends across one or more countries.
As contrasted to satellite communication systems, cellular wireless communication networks typically include a plurality of base stations, each of which communicates with a plurality of mobile stations operating within the cell it supports. Because of the cellular network architecture, a given communication channel (at a particular frequency) may be reused or reassigned for by multiple base stations in geographically distant cells. The concept of common channel allocation (channel reuse) is applicable to AMPS networks where given frequency assignments are made, to TDMA networks where given time slices at specified frequencies are made and CDMA networks where code assignments are made to allow a mobile station to decode a communication.
In satellite systems, however, channels cannot be reused since no geographic separation exists between the subscriber units relative to the satellite/network control center. Accordingly, only one mobile station may be assigned a given channel at any time. Accordingly, certain desirable communication modes that are common in terrestrial cellular networks are problematic in satellite based networks.
By way of example, multicast messaging is currently provided by many communication systems. In providing the multicast messaging service, confirmation of receipt of the multicast messages by subscribing mobile stations is desirable. In cellular based wireless communication systems, confirmation of receipt is easily performed since each base station services only a limited number of subscriber units. However, in a satellite communication system, acknowledgment would result in too many subscriber units attempting to respond on a control channel to a network control center. These responses would be unintelligible to the network control center. Acknowledged multicast messaging in a satellite communication system would have to be performed on a traffic channel and would consume significant available resources. Delivery of multicast messages within a satellite system therefore either (1) consumes significant channel resources; or (2) must be performed without acknowledgment. Neither of these solutions is satisfactory in supporting multicast messaging.
Thus, there exists a need in the art for a satellite based communication system in which multicast messages requiring acknowledgment may be made in a way that confirms delivery of the messages but that also minimizes the consumption of valuable channel resources.